The processing of material, such as semiconductor wafers, for the manufacture of microelectronic circuits involves processing tools for performing a large number of processing steps. The processing steps are usually performed in a vacuum chamber. The processing tools typically handle and process wafers one at a time in order to optimize control and reproducibility. Such processing tools utilize automated wafer handling systems.
The throughput of the processing tools is an important factor in achieving low cost manufacture. The overall throughput is a function of both the processing time and the efficiency of automated wafer handling. Wafer handling involves introduction of the wafers in a wafer carrier into the processing tool, transfer of the wafers from the wafer carrier to a processing station, return of the wafers to the wafer carrier following processing and removal of the wafer carrier from the processing tool. Wafer processing is performed in a vacuum chamber. Wafer handling systems usually include one or more load locks for transferring wafers to and from the vacuum chamber with little impact on the pressure level in the vacuum chamber. The wafer carrier may be a FOUP (Front Opening Unified Pod), which is a standardized wafer carrier utilized for transporting wafers in fabrication facilities, or a cassette. Some of the processing and wafer handling operations may be performed concurrently to achieve efficient operation and high throughput. Accordingly, careful design of wafer handling systems is required. A variety of wafer handling techniques are known in the prior art.
In one prior system disclosed in U.S. Pat. No. 5,486,080, issued Jan. 23, 1996 to Sieradzki, a pair of robot arms transfers wafers from a first cassette in a first load lock to a transfer station and then to a processing station. After wafers from the first cassette have been processed, the robots reverse their respective roles and begin processing wafers from a second cassette located in a second load lock, while the first load lock is vented and the first cassette is replaced with a new cassette.
In another prior art system, a buffer is utilized to transfer wafers from several FOUPs to the load locks. The buffer includes a robot in a controlled environment chamber for transferring wafers to and between the FOUPs and the load locks.
All of the known prior art wafer handling systems have had one or more drawbacks, including but not limited to relatively low throughput, high cost and complex design. Accordingly, there is a need for improved methods and apparatus for transferring of workpieces, such as semiconductor wafers, to and from a vacuum chamber.